Cement-based waterproof coatings are mainly used to prevent water ingression. The waterproofing coatings are brushed or rolled on walls and floors before any decorative layer, such as ceramic tiles, plasters, renders etc., is applied to the surface. After drying, the waterproofing coatings form a dense film which can resist water penetration into the underlying substrates. There have been numerous attempts to improve the performance of waterproofing coatings. One major improvement has been the use of latex modified cement slurries in which a polymeric latex material is added to a cement mortar.
The polymeric latex composition contributes adhesion, tensile strength and elongation properties to the cement-based system. The use of latex polymers in cement systems across a wide variety of concentration levels allows for the production of waterproofing coatings having a wide range of properties. Low levels of latex in cement systems can also improve bonding performance with substrates, especially after rigorous curing conditions, like heat curing, alkali curing etc. Very high levels of latex results in products that exhibit elastomeric qualities. Examples of latex polymers include styrene acrylic, styrene butadiene rubbers, and copolymers of vinyl esters and ethylene. Specifically, copolymers of vinyl acetate and ethylene, (VAE copolymers), are well known as binders that can be added to cement mortar to form cement-based waterproofing coatings. The cement-based waterproofing coating may be used in kitchens or bathrooms by consumers or for exterior walls, basements, roofs and swimming pools by contractors or construction companies.
Typically, a cement-based waterproofing coating includes a liquid component and a solid component, which are mixed together by the consumer or end-user prior to applying the waterproofing coating to the desired surface. The liquid component of the cement-based waterproofing coating includes emulsion copolymers, additional water and plasticizers, such as dibutyl phthalate, (DBP), and dioctyl phthalate, (DOP). The plasticizer is added to improve the elongation and therefore flexibility of the waterproofing coating to avoid cracks. However, it has been reported that plasticizers are potentially harmful to the environment and to humans. Thus, there is interest in formulating waterproofing coatings in which the need for plasticizers is reduced or avoided.
In addition, in an increasing number of applications, industry standards and/or government regulations require that emulsion copolymers be very low in residual monomer content and content of other volatile organic compounds (VOCs). One VOC component which is frequently singled out for specific control is formaldehyde, since formaldehyde is suspected to be a human carcinogen and can be produced by decomposition of some components which are conventionally used in emulsion polymerization processes and products.
Formaldehyde-generating components can include, for example, commonly employed self cross-linking co-monomers such as N-methylolacrylamide. Free formaldehyde can also be released by certain reducing agents such as sodium formaldehyde sulfoxylate used in polymerization initiator systems. Certain emulsion stabilizers used, including, for instance, some types of ethylene oxide-containing surfactants can also form formaldehyde upon oxidation. Formaldehyde formation from polyoxyethylene nonionic surfactants is described, for example, in Bergh et al; Contact Dermatitis, Vol. 39, pp. 14-20 (1998).
One method for reducing the free formaldehyde content of copolymer emulsions is to introduce a formaldehyde binding or scavenging agent. U.S. Pat. Nos. 4,472,165; 4,525,492 and 5,143,954 and European Patent No. 647,658 B1 disclose the use of urea, amines and numerous derivatives of such materials as formaldehyde binders or scavengers in various resin products. Commercially available products, such as Celvolit® 149 LV formerly marketed by Celanese, have used urea as a formaldehyde scavenger.
Another approach to overcoming or ameliorating the effects of free formaldehyde released by N-methylol-based self-crosslinking co-monomers in emulsion copolymer dispersions focuses on the redox initiator systems used to prepare such dispersions. In particular, formaldehyde-generating reducing agents in such redox systems can be replaced with other types of reducing agents which do not generate formaldehyde. U.S. Pat. No. 5,540,987, for example, discloses a redox initiator system for vinyl acetate/N-methylol crosslinking co-monomer emulsion binders, which initiator system reduces the free formaldehyde content of the resulting copolymer emulsion. The redox system of the '987 patent comprises a hydrophobic hydroperoxide oxidizing agent in combination with an ascorbic acid or derivative thereof as a reducing agent.
Similar technology is disclosed in U.S. Pat. No. 6,787,594. The '594 patent describes reduced formaldehyde nonwoven binders based on emulsifier-stabilized vinyl acetate/ethylene emulsion copolymers. Such copolymers also contain an N-methylol-based crosslinking co-monomer. Formaldehyde reduction in the binders of the '594 patent is provided by using a selected type of initiator system during polymerization of the emulsion copolymer. In particular, the initiator system disclosed in the '594 patent is a redox system comprising an oxidizing agent, such as a hydrophobic peroxide, and a reducing agent which is a glycol adduct of sodium sulfite. Particularly preferred reducing agents are said to be sulfinic acid compounds such as 2-hydroxy-2-sulfinato-acetic acid-disodium salt. Reducing agents of this preferred type are sold, for example, under the tradename Bruggolite® FF-6.
Another reference which discloses preparation of emulsion polymers using a similar initiator system, and generally also using emulsifiers as stabilizing agents, is U.S. Pat. No. 6,696,519. The '519 patent utilizes a redox initiator system comprising a water-soluble oxidizing agent, a water-insoluble oxidizing agent and a reducing agent derived from sulfinic acid. The '519 patent is concerned with reducing residual monomer levels in the resulting emulsion and does not mention preparation of low formaldehyde emulsion polymers.
Notwithstanding there remains a need for an improved cement-based waterproofing product that is environmentally friendly, i.e., substantially free of formaldehyde and plasticizer.
According to the present invention it has now been found that the improved plasticizer free cement-based waterproofing product may be made by modifying the manufacturing process of the copolymer emulsion. The stabilizing system comprising at least one medium molecular weight polyvinyl alcohol and at least one low molecular weight polyvinyl alcohol beneficially achieves the tensile strength and elongation that meet industry requirements without the use of plasticizer. In addition, a redox-system comprising a sulfinic acid-based reducing agent, either alone or in combination with erythorbic acid or its alkali metal salt as a reducing agent, and t-butyl hydroperoxide, as an oxidizing agent, is an effective formaldehyde-free initiator for the emulsion polymerization of a vinyl ester/ethylene monomer mixture. Therefore, a formaldehyde and plasticizer free cement-based waterproofing coating made in accordance with the present invention may fulfill industry standards and government regulation at higher filler loading levels and with better applicability than current systems.